1. Field of the Invention
The present invention relates to an improved process for producing biphenylene sulfide polymer.
2. Description of the Prior Art
Because of their outstanding chemical, heat and electrical insulation resistance as well as their superior rigidity when compared to other thermoplastics, poly(arylene sulfide) resins have found favor for forming reinforced plastic composites.
A particularly suitable poly(arylene sulfide) resin for forming fiber-reinforced composites, both by injection molding and pultrusion techniques, is poly(phenylene sulfide) resin. Fiber-reinforced (phenylene sulfide) resin composites exhibit excellent mechanical properties and are utilized in a great variety of products. However, in product applications where very high temperature exposure is required, e.g., temperatures in the range of from about 400.degree. F. to about 600.degree. F., poly(phenylene sulfide) composites generally do not maintain high rigidity and strength. The same problem is experienced in using poly(phenylene sulfide) resin in coatings exposed to high temperatures as well as in other high temperature applications.
A poly(arylene sulfide) resin which has heretofore been found to have a very high melting point and to be suitable for use at very high temperatures is comprised of biphenylene sulfide polymers. Poly(biphenylene sulfide) resin retains its hardness at temperatures as high as about 700.degree. F. whereas poly(phenylene sulfide) resin loses hardness at temperatures in the range of from 300.degree. F. to 400.degree. F. In addition to having a high melting point, poly(biphenylene sulfide) resin has a high retention of mechanical properties at elevated temperatures. For example, composites comprised of reinforcing fibers in a continuous matrix of poly(biphenylene sulfide) resin generally maintain full rigidity and strength (compared to their rigidity and strength at room temperature) up to about 400.degree. F. At 600.degree. F., the composites generally retain about 80% and 35% of their room temperature flexural modulus and flexural strength, respectively.
Heretofore, biphenylene sulfide polymer has been prepared from sodium sulfide and either 4,4'-dichlorobiphenyl or 4,4'-dibromobiphenyl. The resin prepared from 4,4'-dichlorobiphenyl is superior to that which is prepared from 4,4'-dibromobiphenyl. However, the use of polychlorinated biphenyl such as 4,4'-dichlorobiphenyl has been discontinued as a result of its having been found to be hazardous to the environment and human health.
By the present invention, an improved process for producing biphenylene sulfide polymer which has a higher melting point as compared to biphenylene sulfide resin formed from sodium sulfide and 4,4'-dibromobiphenyl is provided. As a result, fiber reinforced composites and coatings formed using the polymer have better mechanical properties at elevated temperatures.